Direct methanol fuel cells (DMFCs), fuel cells in which the anode is fed directly with liquid or vaporous methanol, have been under development for a considerable period of time, and are well-known in the art. See for example Baldauf et al, J. Power Sources, vol. 84, (1999), Pages 161-166. One essential component in a direct methanol, or any, fuel cell is the separator membrane.
It has long been known in the art to form ionically conducting polymer electrolyte membranes and gels from organic polymers containing ionic pendant groups. Well-known so-called ionomer membranes in widespread commercial use are Nafion® perfluoroionomer membranes available from E. I. du Pont de Nemours and Company, Wilmington Del. Nafion® is formed by copolymerizing tetrafluoroethylene (TFE) with perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), as disclosed in U.S. Pat. No. 3,282,875. Other well-known perfluoroionomer membranes are copolymers of TFE with perfluoro (3-oxa-4-pentene sulfonyl fluoride), as disclosed in U.S. Pat. No. 4,358,545. The copolymers so formed are converted to the ionomeric form by hydrolysis, typically by exposure to an appropriate aqueous base, as disclosed in U.S. Pat. No. 3,282,875. Lithium, sodium and potassium are all well known in the art as suitable cations for the above cited ionomers.
Other fluorinated ionomer membranes are known in the art such as those described in WO 9952954, WO 0024709, WO 0077057, and U.S. Pat. No. 6,025,092.
DMFCs employing ionomeric polymer electrolyte membranes as separators are known to exhibit high methanol cross-over—the transport of as much as 40% of the methanol from the anode to the cathode by diffusion through the membrane. This methanol cross-over essentially represents a fuel leak, greatly decreasing the efficiency of the fuel cell. In addition, the presence of methanol at the cathode interferes with the cathode reaction, with the methanol itself undergoing oxidation, and, in sufficient volume, floods the cathode and shuts down the fuel cell altogether. Methanol cross-over occurs primarily as a result of the high solubility of methanol in the ionomeric membranes of the art.
It is of considerable interest in the art to identify ways to reduce methanol cross-over in ionomeric membranes while entailing as small as possible cost in conductivity.
Kyota et al, JP Sho 53(1978)-60388, describes a process for producing modified Nafion® membranes with reduced permeability to hydroxide ion by swelling with a solvent or liquid, diffusing a polymerizable vinyl monomer into the swollen matrix with an initiator, and polymerizing in situ. Also disclosed by reference is a process for diffusing the monomers without solvent-swelling, but the solvent-swelling process is said to be superior. Methanol permeability is not discussed.
Seita et al, U.S. Pat. No. 4,200,538, disclose a cation exchange membrane prepared by swelling a fluorinated ionomer with an organic solvent, removing the solvent, immersing in a vinyl monomer, adding initiators and other additives, and polymerizing the monomer in situ. Improvements in hydroxyl ion permeability are noted. Suitable monomers include styrene and styrene derivatives; acrylic, methacrylic, and maleic acids and salts and esters thereof; vinyl acetate, vinyl isocyanate, acrylonitrile, acrolein, vinyl chloride, vinylidene chloride, vinylidene fluoride, vinyl fluoride; and numerous others. Methanol permeability is not discussed.
Fleischer et al, U.S. Pat. No. 5,643,689, disclose composite membranes which include combination of ionomeric polymers and numerous non-ionic polymers including polythyleneimine and polyvinylpyrrolidone. Metal oxides are present in the composite. The composites are prepared by dissolving the respective polymers in a common solvent and then removing the solvent, and are said to be useful in hydrogen fuel cells.
Li et al, WO 98/42037, discloses polymer electrolyte blends in batteries. Disclosed are blends of polybenzimidazoles with Nafion® and other polymers in concentration ratios of ca. 1:1. Preferred are blends of polybenzimidazoles and polyacrylamides. Polyvinylpyrrolidone and polyethyleneimine are also disclosed.